The present application relates to nonaqueous electrolyte secondary batteries, and in particular, to a lithium ion secondary battery using a lithium complex oxide as an electrode material.
In recent years, with the remarkable progress of mobile electronics technology, mobile electronics devices such as cellular phones and notebook PCs are increasingly recognized as fundamental technologies supporting the advanced information-oriented society. Further, research and development aimed at sophistication of these devices are being vigorously made, simultaneously electric power consumption of electron devices is growing steadily. On the other hand, these electronics devices should work for long hours, consequently secondary batteries for driving these devices are desired to have higher energy density.
The energy density of a battery is preferably higher from the viewpoints of occupying volume and weight of the battery in electronics devices. For satisfying the demand, at present, nonaqueous electrolyte secondary batteries, in particular, lithium ion secondary batteries utilizing doping/dedoping of lithium ions are included in most devices due to their excellent energy density.
In common cases, a lithium ion secondary battery has, for example, a cathode including a cathode collector having formed thereon a cathode active material layer using a lithium complex oxide such as lithium cobaltate, and, for example, an anode including an anode collector having formed thereon an anode active material layer using a carbon material, and is used at an operating voltage of 2.5 V to 4.20 V. The terminal voltage of a single cell can be increased up to 4.20 V largely owing to the excellent electrochemical stability of the nonaqueous electrolyte material, separator, and others.
However, in a lithium ion secondary battery as described above which operates at a maximum voltage of 4.20 V, the entire theoretical capacity of the cathode active material contained therein such as lithium cobaltate is not thoroughly utilized for discharging and charging, and only about 60% of the capacity is utilized. Therefore, for further improving the battery characteristic of a secondary battery, a lithium ion secondary battery described in International Publication No. WO 03/019713 has an increased charge cutoff voltage of 4.25 V or higher.
It is known that the above-described battery has a charged voltage of 4.25 V or higher, which increases the amount of doped/dedoped lithium between the layers of a carbon material to impart a larger capacity and higher energy density to the lithium ion secondary battery.
On the other hand, a higher energy density tends to raise the peak temperature inside the battery in case of abnormal heat generation. When a short circuit current is passed through a battery because of misuse or destructive test, Joule heat is generated. The amount of such heat is further growing in the development for increasing the energy density of the past.
Heretofore, large-capacity lithium ion secondary batteries have been improved in their characteristics and reliability by adding aluminum or the like to the battery system.
For example, a battery described in Japanese Patent No. 3552361 prevents the influences of internal short-circuit in a large-capacity lithium ion secondary battery from extending to between adjacent cathode and anode, and prevents direct short-circuit of the cathode and anode.
In Japanese Patent No. 3552361, an interface where the cathode is not opposed to the anode is provided, and a heat resistant layer is provided on the interface by spraying powder of a metal oxide such as aluminum oxide (Al2O3) by, for example, plasma spraying. The heat resistant layer prevents the influences of internal short-circuit from extending to between the adjacent cathode and anode. In addition, even when fusion or decomposition of the separator is caused by heat, the layer secures electrical insulation between the cathode and anode, and prevents direct short-circuit of the cathode and anode.
However, the layer of an anode active material during charging may cause exothermic reaction when subjected to heat generated by abnormal use or misuse, or other electric heat generated in the battery system. Therefore, the heat resistant layer provided on the interface between the cathode and anode contributes to stabilize the inside of the battery, but does not decrease the temperature in the battery in case of abnormal heat generation.
Besides, the addition of aluminum oxide or other substances devoid of active material capacity to a battery system causes the decrease in the volume efficiency, and the increase in resistance and thus deteriorates the load characteristic, which makes it difficult to strike a balance between battery characteristic and reliability.